This invention relates to an internal combustion engine, and in a preferred embodiment of the invention provides a small single cylinder internal combustion engine of high efficiency suitable for use, for example, in a motor cycle or to drive a small generator, pump or the like. Whilst the preferred embodiment of the invention is concerned with such small engines, and in particular such engines of single cylinder design, it is to be understood that the invention is not limited to such designs and features of the invention may be applicable to multi-cylinder engines and to engines of relatively large size.
For the purposes of this application it will be assumed that the engine in question is oriented such that the longitudinal axis of the cylinder is vertical and the axis of rotation of the crankshaft is horizontal with the said axes intersecting at a point vertically below the geometric centre of the piston. With such an orientation the cylinder head is above the piston and when viewed from above the cylinder head will lie between the viewpoint and the crown of the piston. It should be appreciated that in practice engines in accordance with embodiments of the invention need not have this orientation, but this orientation will be used in this description since it provides a convenient method of defining the relative positions of the various components of the engine.
In a conventional overhead valve engine each cylinder has one inlet valve and one exhaust valve. Each valve has associated therewith a valve seat. An inlet port connects a source of combustion air (usually admixed with fuel) to the inlet valve seat and an exhaust port connects the exhaust valve seat to an exhaust outlet which normally has connected thereto an exhaust pipe and silencer arrangement. The inlet and exhaust ports have longitudinal centre lines which, when viewed from above, are generally straight. When viewed from the sides, the inlet and exhaust ports generally have a major portion which is straight although there may be some change of direction of the port immediately adjacent the valve seat. Even if there is some local variation in the shape of the ports adjacent the valve seats, the inlet and exhaust ports generally have a substantially straight longitudinal centre lines when viewed from the side over the majority of their length.
The position of the longitudinal centre lines of the inlet and exhaust ports can be defined by two angles, the first (hereinafter referred to as the side angle) being the angle which the longitudinal centre line, when projected onto a plane perpendicular to the longitudinal axis of the cylinder, makes relative to an imaginary line passing through the geometric centres of the inlet and exhaust valve seats when that imaginary line is also projected onto the said plane. The other angle (hereinafter referred to as the vertical angle) is the angle which the longitudinal centre line of the port makes to the said plane. For the purposes of this specification the side angle of a port is considered to be 0xc2x0 when the projection of the longitudinal centre line of the port onto the said plane is coincident with the projection onto that plane of the imaginary line passing through the geometric centres of the inlet and exhaust valves and overlies that portion of the imaginary line which is located on the side of the valve seat associated with the port in question which is remote from the other valve seat. The side angle is always considered to be positive, i.e. the side angle of a port is stated to be Xxc2x0 if the port is rotated Xxc2x0 clockwise or Xxc2x0 anti-clockwise from the 0xc2x0 position. It follows from this that the maximum possible value of side angle is 180xc2x0, and this occurs when the projection of the longitudinal centre line of the port onto a plane perpendicular to the longitudinal axis of the cylinder is coincident with and overlies that portion of the imaginary line passing through the geometric centres of the valve seats which is located between the geometric centres of the valve seats.
For a general review of the widely accepted understanding of the significance of the various characteristics of small internal combustion engines attention is directed to Chapter 9 of MOTOR CYCLE ENGINEERING by P. E. Irving published by Clymer Publications of Los Angeles, Calif., U.S.A. (ISBN 0-85113-075-5). It will be noted from this publication that it is well recognized that a high level of turbulence or swirl within the combustion chamber is desirable to ensure complete combustion of the fuel in a motor cycle sized internal combustion engine. It is further recognized that the desired swirl can be achieved by angling the inlet port relative to the imaginary line passing through the geometric centres of the inlet and exhaust valve seats in a manner which produces the required level of swirl i.e. giving the inlet port a side angle of greater than 0xc2x0. It is further recognized that, whilst increasing the side angle of the inlet port increases the amount of swirl with the result that high levels of torque are produced at low and medium speeds, as the side angle is increased the volumetric efficiency of the engine decreases resulting in limited performance at high revs and accordingly limited maximum power output. In fact, in many modern engines, for example the Honda GX160, a side angle of substantially 0xc2x0 is used for the inlet port notwithstanding the well recognised advantages of using an inlet port side angle of greater than 0xc2x0 to increase cylinder swirl. It is generally recognised that the side angle of the inlet port should not exceed 18xc2x0.
I have now discovered that improved performance can be obtained if the inlet port side angle is substantially greater than would be considered acceptable in conventional engine design.
Accordingly, one aspect of the present invention provides an internal combustion engine having a cylinder; a piston reciprocable within the cylinder; a cylinder head closing the top of the cylinder; an inlet valve mounted in the cylinder head; an inlet valve seat against which the inlet valve closes; an exhaust valve mounted in the cylinder head; an exhaust valve seat against which the exhaust valve closes; a source of combustion air; and an inlet port connecting the source of combustion air to the inlet valve seat, wherein the side angle of the inlet port is greater than 60xc2x0.
The cylinder head may be formed separately from the cylinder block and secured thereto by conventional means, or may be integral with the cylinder block.
In the preferred embodiment of the invention the inlet port side angle is in the range 60xc2x0-160xc2x0. For the avoidance of doubt, the disclosure herein of the range 60xc2x0-160xc2x0 is to be construed as a disclosure of every range having a lower limit between 60xc2x0 and 160xc2x0 and an upper limit of between 60xc2x0 and 160xc2x0.
It has been found that the optimum inlet port side angle is in the range 90xc2x0 to 112xc2x0 and more particularly in the range 95xc2x0-107xc2x0 with a preferred value of 101xc2x0xc2x13xc2x0. The inlet port vertical angle can be varied depending on the desired maximum speed of the engine. For slow running engines a relatively small vertical angle will produce optimum results whilst a higher vertical angle will provide improved results at high revs. The preferred vertical angle is in the range of 10xc2x0 to 60xc2x0. Different embodiments of engines according to the present invention have been found to work satisfactorily with inlet port vertical angles of 24xc2x0xc2x13xc2x0 and 55xc2x0xc2x13xc2x0.
The exhaust port angles are less critical to performance of the engine than the inlet port angles. In the preferred embodiment of the invention the exhaust port side angle is in the range 90xc2x0-95xc2x0 with a preferred value of 93xc2x0 and the exhaust valve vertical angle is in the range 3xc2x0-23xc2x0 with a most preferred value of 13xc2x0.
It has been found that the performance of the engine can be further enhanced if the inlet and exhaust valve seats are placed asymmetrically relative to the longitudinal axis of the cylinder. In particular, in one preferred arrangement the inlet valve is located closer to the longitudinal axis of the cylinder than the exhaust valve. In a particularly preferred embodiment of the invention the inlet valve is positioned such that the most central portion of the inlet valve seat lies substantially on the projected longitudinal axis of the cylinder whilst the nearest point of the exhaust valve seat to the said projected longitudinal axis is displaced some distance from the projected longitudinal axis. It has also been found that performance of the engine is enhanced if the inlet valve is offset from the diameter of the cylinder which passes through the geometric centre of the exhaust valve. The effect of this arrangement is such that the imaginary line passing through the geometric centres of the inlet valve seat and the exhaust valve seat is parallel to but displaced to one side of a reference diameter of the cylinder. Preferably, the inlet port approaches the inlet valve seat from the side of the inlet valve seat remote from this reference diameter. The effect of this arrangement, in combination with the large inlet port side angle of the invention, is that the combustion air enters the cylinder generally tangentially to the cylinder wall. This produces a high level of swirl and, it is believed, leads to stratification of the fuel air mixture with the result that a fuel rich mixture is created in the central region of the cylinder, this fuel rich region being surrounded by a region which is fuel lean or even substantially devoid of fuel. It is believed that this stratification effect contributes at least in part to the excellent operating characteristics of the embodiments of engine according to the present invention.
If the believed stratification effect occurs it follows that enhanced combustion will be achieved if the sparking plug is located as close as possible to the geometric centre of the fuel rich zone. This will typically be close to the longitudinal axis of the cylinder. Positioning the sparking plug at this point should reduce the burn time for the fuel, thereby reducing the amount of ignition advance required and reducing the Nox emissions of the engine.
The advantages of the present invention are particularly apparent when the engine is of the xe2x80x9cover squarexe2x80x9d type, that is to say the diameter of the cylinder is greater than the stroke of the piston. Such an arrangement enables relatively large sized valves to be accommodated without having to angle the valves relative to the longitudinal axis of the cylinder. In other words, with an over square engine the valves can be mounted so that their longitudinal stems are parallel to the longitudinal axis of the cylinder. The high degree of swirl and high volumetric efficiency produced by the port designs according to the present invention enable the engine to operate efficiently without the conventional angling of the valves. This leads to easy and cost effective manufacture of the cylinder head. The inlet and exhaust valves can be identical which further reduces manufacturing costs.
Engines manufactured in accordance with the present invention have been found to achieve very low emission levels of carbon monoxide. For example, in one test engine carbon monoxide emission levels of 0.06% have been achieved. These low carbon monoxide levels are accompanied by very low levels of unburnt hydrocarbon, for example 4 parts per million, and good levels of CO2 emission. The engine has also to be found to be tolerant of low octane fuels since it is resistant to fuel detonation. The exhaust of test engines has also been found to have high levels of oxygen suggesting that the preferred inlet port geometry leads to a degree of stratification whereby fuel is concentrated in the central portion of the combustion chamber. The exhaust emissions from test engines were also found to be at low temperature. Test engines have been found to give good power output and low specific fuel consumption.
The invention will be better understood from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawings.